To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post LTE system’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like.
In the 5G system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
Long-Term Evolution (LTE) system of 3rd Generation Partnership Project (3GPP) supports two duplex modes including Frequency Division Duplex (FDD) and Time Division Duplex (TDD).
As shown in FIG. 1 which shows an existing FDD radio frame structure, for the FDD system, each radio frame is of 10 ms length, consists of ten 1 ms subframes. Each subframe consists of two consecutive 0.5 ms slots, i.e., the kth subframe includes slot 2k and slot 2k+1, k=0, 1, . . . , 9.
As shown in FIG. 2 which shows an existing TDD radio frame structure, for the TDD system, each 10 ms radio frame is divided into two 5 ms half-frames. Each half-frame includes eight 0.5 ms subframes and three special fields, i.e., downlink pilot slot (DwPTS), guard period (GP) and uplink pilot slot (UpPTS). The total length of the three special fields is 1 ms. Each subframe consists of two consecutive slots, i.e., the kth subframe includes slots 2k and slot 2k+1, k=0, 1, . . . , 9. One downlink Transmission Time Interval (TTI) is defined in one subframe.
When the TDD radio frame is configured, 7 kinds of uplink-downlink configurations are supported, as shown in Table 1. Herein, D denotes downlink subframe, U denotes uplink subframe, and S denotes a special subframe including the above three special fields.
Table 1 uplink-downlink configurations of LTE TDD.
TABLE 1Config-SwitchingurationpointSubframe indexindexperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
First n Orthogonal Frequency Division Multiplexing (OFDM) symbols of each downlink subframe may be used for transmitting downlink control information. The downlink control information includes Physical Downlink Control Channel (PDCCH) and other control information, wherein n=0, 1, 2, 3 or 4; remaining OFDM symbols may be used for transmitting Physical Downlink Shared Channel (PDSCH) or Enhanced PDCCH (EPDCCH). In the LTE system, PDCCH and EPDCCH are used for bearing Downlink Control Information (DCI) allocating uplink channel resources or downlink channel resources, respectively referred to as Downlink Grant (DL Grant) and Uplink Grant (UP Grant). In the LTE system, the DCI of different UEs are transmitted independently, and the DL Grant and the UL Grant are also transmitted independently.
In enhanced system of the LTE system, a wider working bandwidth is obtained through combining multiple Component Carriers (CC), i.e., Carrier Aggregation (CA), to form downlink and uplink of the communication system and therefore support higher transmission rate. Herein, the aggregated CCs may have the same duplex mode, e.g., all of them are FDD cells or TDD cells. The aggregated CCs may also have different duplex modes, i.e., there exist FDD cells and TDD cells at the same time. For one UE, a base station may configure it to work in multiple cells, one of them is a primary cell (PCell) and others are secondary cells (SCell). For the LTE CA system, transmission of the Hybrid Automatic Repeat Request-Acknowledgement (HARQ-ACK) and Channel State Information (CSI) on the Physical Uplink Control Channel (PUCCH) are only implemented on the Pcell.
The above LTE system is usually deployed on a licensed band, so as to avoid interferences from other systems. Besides the licensed band, there are also unlicensed bands. The unlicensed bands generally have been allocated for other usages, e.g., radar system and/or 802.11 series WiFi system. The 802.11 series WiFi system operates based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Before transmitting signals, a Station (STA) needs to sense the wireless channel. The wireless channel can be occupied for transmitting signals only when it has been idle for a specified time period. The STA may determine the status of the wireless channel according to two mechanisms in association. On the one hand, the STA may practically sense the wireless channel using the carrier sensing technique. If signals of another STA are detected or perceived signal power is higher than a predefined threshold, it is determined that the wireless channel is busy. At this time, a physical layer module of the STA may report a Clear Channel Assessment (CCA) report to a higher layer module indicating that the wireless channel is busy. On the other hand, a virtual carrier sensing technique is also introduced in the 802.11 series WiFi system, i.e. Network Allocation Vector (NAV). Each 802.11 frame includes a duration field. The STA may determine whether it can transmit signal in the wireless channel according to a NAV value in the duration field, wherein the NAV indicates the amount of time that the wireless channel needs to be reserved.
For the LTE system, in order to meet the increasing demand of the mobile communication services, more spectrum resources need to be explored. One possible solution is to deploy the LTE system on the unlicensed band. Since the unlicensed band have generally been allocated for other usages, the interference level may be unpredictable when the LTE system is deployed on the unlicensed band, which makes it difficult to ensure Quality of Service (QoS) of data transmission of the LTE system. But the unlicensed band still can be used for data transmission with low QoS requirement. In this situation, how to avoid the interference to the LTE system on the unlicensed band has become an urgent problem to be solved.